Newton 2.0 - Collision ID: is this important?

The Question is a simple one: In the new OgreNewt, Collision Primitives have constructors like:
Box( const World* world, const Ogre::Vector3& size, [b]int id[/b], const Ogre::Quaternion& orient = Ogre::Quaternion::IDENTITY, const Ogre::Vector3& pos = Ogre::Vector3::ZERO );
is this id used for anything besides collision->getUserId()? It is in the default (and only) constructor and has no default value, for me this looks like something relatively important. Is it important?
If no, why not make it = 0 by default and/or add a constructor without it? If yes, what should I set it to?

It depends if you need to know a certain instance of an object you created with that particular primitive.

If you don't need to know that then just don't fill that parameter.